Magnetic storage is the most common medium for storing binary and analog data. Re-writeable optical storage has only recently entered the mainstream of computing. The most common system currently in the consumer market is magneto-optical system. Magneto-optical systems record data magnetically and optically in combination and read data optically. Magneto-optical storage provides a more robust storage of binary data that has less degradation of the written data than magnetic storage.
For magnetic recording, data previously recorded on the magnetic medium is erased concurrently with writing data on the medium as a one-pass process. In contrast, for magneto-optical recording, a two-pass process is required for writing data. During a first pass, the magneto-optical medium is reset by erasing data from the medium. During a second pass, the magneto-optical medium is set by writing data to the medium. During the erase pass, the entire region is reset by applying a magnetic field ("north" up) to the medium and then continuous applying high power light from a laser to the medium. In this pass, all data bits are reset to "0". During the write pass, the magnetic field is switched to the opposite polarity ("north" down) and the laser is pulsed to write data only where the specific bits are to be "1". This combination gives the desired binary data (e.g., 0100111).
Unlike magnetic recording systems which allows changes in the magnetic field to occur rapidly, magneto-optical recording systems require a larger magnetic field and have a magnet that is physically further away from the active layer of the recording medium. Changing the magnetic field rapidly in the magneto-optical system, as is done in magnetic recording systems, increases the power requirements and may exceed the physical limitations of the medium. One method for overcoming this problem is to increase the sensitivity of the magneto-optical medium and slow the recording process, but this compromises the data integrity features, significantly limits performance, and is not compatible with conventional recording standards.
Another method for overcoming the two pass process has been to reset the bits before data is written to the disk, in a process commonly known as pre-erase. Pre-erasing the medium is handled by the host computer system through special device drivers and with non-standard calls to the optical disk drive. When the data is overwritten or needs to be erased, the host computer system must slow down or multi-thread to manage the data phase of the drive to thereby increase system overhead. Also, formats other than conventional data structures are required. In addition, because non-standard calls are used, special device drivers are required at a significantly increased cost and overhead. Finally, because there is no margin for hardware or software failure, all data on a disk is at risk during the pre-erase phase.